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Thread-Safe Collections		03/30/2017	.net			dotnet-standard		article	thread-safe collections, overview	2e7ca21f-786c-4367-96be-0cf3f3dcc6bd	24	mairaw	mairaw	wpickett

Thread-Safe Collections

The [!INCLUDEnet_v40_short] introduces the xref:System.Collections.Concurrent?displayProperty=nameWithType namespace, which includes several collection classes that are both thread-safe and scalable. Multiple threads can safely and efficiently add or remove items from these collections, without requiring additional synchronization in user code. When you write new code, use the concurrent collection classes whenever the collection will be writing to multiple threads concurrently. If you are only reading from a shared collection, then you can use the classes in the xref:System.Collections.Generic?displayProperty=nameWithType namespace. We recommend that you do not use 1.0 collection classes unless you are required to target the .NET Framework 1.1 or earlier runtime.

Thread Synchronization in the .NET Framework 1.0 and 2.0 Collections

The collections introduced in the .NET Framework 1.0 are found in the xref:System.Collections?displayProperty=nameWithType namespace. These collections, which include the commonly used xref:System.Collections.ArrayList and xref:System.Collections.Hashtable, provide some thread-safety through the Synchronized property, which returns a thread-safe wrapper around the collection. The wrapper works by locking the entire collection on every add or remove operation. Therefore, each thread that is attempting to access the collection must wait for its turn to take the one lock. This is not scalable and can cause significant performance degradation for large collections. Also, the design is not completely protected from race conditions. For more information, see Synchronization in Generic Collections on the MSDN Web site.

The collection classes introduced in the .NET Framework 2.0 are found in the xref:System.Collections.Generic?displayProperty=nameWithType namespace. These include xref:System.Collections.Generic.List%601, xref:System.Collections.Generic.Dictionary%602, and so on. These classes provide improved type safety and performance compared to the .NET Framework 1.0 classes. However, the .NET Framework 2.0 collection classes do not provide any thread synchronization; user code must provide all synchronization when items are added or removed on multiple threads concurrently.

We recommend the concurrent collections classes in the [!INCLUDEnet_v40_short] because they provide not only the type safety of the .NET Framework 2.0 collection classes, but also more efficient and more complete thread safety than the [!INCLUDEnet_v10_short] collections provide.

Fine-Grained Locking and Lock-Free Mechanisms

Some of the concurrent collection types use lightweight synchronization mechanisms such as xref:System.Threading.SpinLock, xref:System.Threading.SpinWait, xref:System.Threading.SemaphoreSlim, and xref:System.Threading.CountdownEvent, which are new in the [!INCLUDEnet_v40_short]. These synchronization types typically use busy spinning for brief periods before they put the thread into a true Wait state. When wait times are expected to be very short, spinning is far less computationally expensive than waiting, which involves an expensive kernel transition. For collection classes that use spinning, this efficiency means that multiple threads can add and remove items at a very high rate. For more information about spinning vs. blocking, see SpinLock and SpinWait.

The xref:System.Collections.Concurrent.ConcurrentQueue%601 and xref:System.Collections.Concurrent.ConcurrentStack%601 classes do not use locks at all. Instead, they rely on xref:System.Threading.Interlocked operations to achieve thread-safety.

[!NOTE] Because the concurrent collections classes support xref:System.Collections.ICollection, they provide implementations for the xref:System.Collections.ICollection.IsSynchronized%2A and xref:System.Collections.ICollection.SyncRoot%2A properties, even though these properties are irrelevant. IsSynchronized always returns false and SyncRoot is always null (Nothing in Visual Basic).

The following table lists the collection types in the xref:System.Collections.Concurrent?displayProperty=nameWithType namespace.

Type	Description
xref:System.Collections.Concurrent.BlockingCollection%601	Provides bounding and blocking functionality for any type that implements xref:System.Collections.Concurrent.IProducerConsumerCollection%601. For more information, see BlockingCollection Overview.
xref:System.Collections.Concurrent.ConcurrentDictionary%602	Thread-safe implementation of a dictionary of key-value pairs.
xref:System.Collections.Concurrent.ConcurrentQueue%601	Thread-safe implementation of a FIFO (first-in, first-out) queue.
xref:System.Collections.Concurrent.ConcurrentStack%601	Thread-safe implementation of a LIFO (last-in, first-out) stack.
xref:System.Collections.Concurrent.ConcurrentBag%601	Thread-safe implementation of an unordered collection of elements.
xref:System.Collections.Concurrent.IProducerConsumerCollection%601	The interface that a type must implement to be used in a BlockingCollection.

Related Topics

Title	Description
BlockingCollection Overview	Describes the functionality provided by the xref:System.Collections.Concurrent.BlockingCollection%601 type.
How to: Add and Remove Items from a ConcurrentDictionary	Describes how to add and remove elements from a xref:System.Collections.Concurrent.ConcurrentDictionary%602
How to: Add and Take Items Individually from a BlockingCollection	Describes how to add and retrieve items from a blocking collection without using the read-only enumerator.
How to: Add Bounding and Blocking Functionality to a Collection	Describes how to use any collection class as the underlying storage mechanism for an xref:System.Collections.Concurrent.IProducerConsumerCollection%601 collection.
How to: Use ForEach to Remove Items in a BlockingCollection	Describes how to use foreach, (For Each in Visual Basic) to remove all items in a blocking collection.
How to: Use Arrays of Blocking Collections in a Pipeline	Describes how to use multiple blocking collections at the same time to implement a pipeline.
How to: Create an Object Pool by Using a ConcurrentBag	Shows how to use a concurrent bag to improve performance in scenarios where you can reuse objects instead of continually creating new ones.

Reference

xref:System.Collections.Concurrent?displayProperty=nameWithType